One-sided covered marking on sheet metal surfaces

ABSTRACT

The object of the process is to hide a visible and legible mark ( 7 ) impressed on one side of a thin metal sheet ( 6   a ) used for making beverage cans in such a way that an image does not show on the reverse side. A second impression ( 10; 11; 12; 13 ) is embossed on the reverse side ( 6   b ) in such a way that the visibility and legibility of the mark ( 7 ) is retained on the one side, but the image on the reverse side is covered and made illegible.

[0001] The invention relates to a method for hiding marks on thin metalsheets or similar surfaces, especially in two part beverage cans, aswell as such marked metal sheets.

[0002] It often happens that distinguishing marks or other codes areembossed (or impressed in some other way) on the thin sheets used formaking boxes and lids, especially for drinking liquids, The preferredmethod is stamping, or scratch impressions. One disadvantage of thismethod is that an image of the mark, sign, or code appears on thereverse side of the thin metal. Although a mirror image, it can bedeciphered easily. This is often not desirable.

[0003] The object of the invention is to suggest a method by whichcovered marking is provided on thin sheets wherein formation of a markimpressed on one side, preferably embossed or otherwise, of a thin metalsheet. The image appearing on the other side of the metal sheet is notto appear according to the invention.

[0004] This is achieved by the method according to claim 1 or 14. It isprovided that the reverse (other) side of the thin metal showing theimage is subsequently subjected to a second embossing process in such away that the visibility and legibility of the embossed mark on one sideis unimpaired, while at the reverse side of the sheet the image iscovered by the subsequent embossing and cannot be read or deciphered anymore.

[0005] The subsequent embossing is carried out by means of a speciallysurface structured stamp which provides a textural, patterned or hatchedimpression (embossing) on the reverse side of the metal sheet which thenreliably covers the image.

[0006] The original impression can, for example, be applied to theinside of a thin metal lid with its inside surface turned to theinterior of the closed can. The second stamping process is then appliedto the outside of the lid that is part of the visible surface of theclosed can.

[0007] It often happens that a tear-out tab end can be opened with ashaped tab of thin sheet metal. An advantage of this is that theembossment or impression (which penetrates the surface) could be placedon the bottom side of the removable tab, thus being invisible (claim17). The image of the mark appearing on the upper surface of the tab canthen be covered by the second impression on the upper surface. The stampused for the second procedure can be textured, patterned or hatched(claim 11).

[0008] The invention can also be applied to a lid made of thin sheetmetal (claim 10) or a section of such a surface (claim 16) which has tobe retained according to claim 1 or claim 14, or is actually retainedafter the embossing process. The steps of the procedure immediatelyindicate how the section of the surface would appear after beingimpressed.

[0009] The embossing processes both of which alter the structure of thesurface differ in depth of penetration and in regard to lateral extent(claim 6). The second impression changes the properties, smoothness, orquality of the surface to hide the image of the first embossment. Thefirst deformation is locally more pronounced and penetrates deeper intothe surface material and is still visible after the second impression.The quality of the surface of the image can be compared with adjoiningsections of the surface not affected by the first embossment.

[0010] Both the surface extension and the smaller depth enable thesecond impression to fulfill its task of hiding the first embossmentwithout changing it (claim 8).

[0011] The “thinness” of the metal sheets means either that we aredealing with material in sheet form, or that the concept thin sheetmetal (German “Blech”) is sufficient to indicate that it actually is athin metal sheet. Although thicker sheet metal is made, the thickness ofthe kind used in boxes for juices is not greater than 0.3 mm (claim 15).The thickness of the metal can also be seen with relation to the depthof penetration of the first impression (claim 5). Only when an imageappears on the reverse side, the metal is regarded as “thin” and thesecond procedure is required to hide it.

[0012] A surface change by means of a covering layer like paint is notreally an altered surface. The outer layer can be regarded as beingjoined to the surface, but it can be removed. However, surfacedeformations produced by pressure, constitute a direct intrusion intothe surface that cannot be removed from the metal sheet or from someother material like PET or plastic.

[0013] The direction of the deformation comprises a further advantage ofimpression by means of vertical pressure which alters the surface, sinceit can easily be implemented when boxes or lids are finished. Transversemotions cannot be applied directly, but such movements are not required.

[0014] The invention results in a clearly legible mark hidden in thefinished product. The advantage of, for example, embossing the mark, isthat it cannot be removed as could be done in the case a printed mark.The impressed mark invariably leaves a mirror image on the visible sideof the metal sheet, but this can be scrambled, guaranteeing that themark is completely hidden. A rough surface is impressed over the image,completely covering it and making it practically unreadable.

[0015] One way of making a “scrambling” stamp is by using spark erosion.This would extend over the reverse side of the first embossment. It isalso possible to apply spark erosion directly to the reverse image(claim 14) as a second surface change opposing the original impression.This process can be compared with sandpaper which penetrates into thesurface thus corresponding to deformation by pressure, or spark erosionwhich removes surface particles; in any case, it constitutes a surfacepenetrating change.

[0016] Experimental examples have shown that penetration depths ofbetween {fraction (1/100)} and {fraction (2/100)} mm can be attainedwith the current metal thicknesses of lids and tab loops. The depth ofpenetration is appreciably greater on the side impressed first, whetherembossing is done by pressing, scratching, carving or cutting. Thisdepth lies between {fraction (4/100)} and {fraction (5/100)} mm.

[0017] Compared to the reflectivity of the smooth, high quality surface,light reflection is destroyed on the roughened section of the surfacewhich is responsible for the visibility of the mark image even if it isnot raised.

[0018] The invention is further explained by means of schematic drawingsof several examples.

[0019] The drawings show:

[0020]FIG. 1 a partial perspective view of an upper section of a can,for beverages for example, which has a tear-out tab with a loop handle.

[0021]FIG. 2 is a partial view from above of the grip end of a loophandle which can be used for a tear-out tab.

[0022]FIG. 3 shows the cutout according to FIG. 2 of the reverse side.

[0023]FIG. 4 is a large-scale view of FIG. 3 after covering the image ofthe mark by means of a second impression according to the procedure.

[0024]FIG. 4a,

[0025]FIG. 4b are cross sections of FIG. 4 illustrating the change ofthe surface after the first impression and the second impression.

[0026]FIG. 5 is a schematic representation of various further possiblemeans of impression in the second process, combined in one image as anegative contour on the reverse side of a schematic stamp 20.

[0027]FIG. 6,

[0028]FIG. 7,

[0029]FIG. 8 indicate sections of actual metal parts.

[0030] A container as shown in FIG. 1 consists of thin sheet metal andit is closed with a lid 3 which also consists of thin metal sheeting.The latter exhibits a seam in the area of the lid tab 3 a, along whichthe tab can be torn out, at least partially, to result in an opening. Tostart and proceed with the tearing open, a loop 5 is provided which isfixed near one end to the lid tab 3 a by means of a rivet 4. A gripsection 6 at the other end can be lifted, torn loose, and pressed intothe opening.

[0031] An enlarged section of one end of the grip 6 of the grip loop 5(see FIG. 1) is shown in FIG. 2 as seen from below. Thus, one seessurface 6 a, the end of the grip loop from below. An embossed mark hasbeen impressed in the surface near the grip end. In this case anumerical code has been carved or scratched into the surface comprisingthree separate systems 7 a, 7 b, and 7 c as arranged in area 3 a′.

[0032] Because the sheet metal used is quite thin, it is inevitable thata tab image 8 of the impression of code 7 (see FIG. 2) is formed on thereverse side. In this case the FIGS. 8a, 8 b and 8 c appear on the upperside 6 b according to FIG. 3 as can be seen in section 3 a′. Althoughthe codes 7 embossed on the grip loop are hidden, being on the undersideof grip section 6, they can be deciphered directly from the outside asshown in FIG. 3.

[0033]FIG. 4 show that this image has been overlain by a textural,patterned, or crosshatched second impression. The visible outer side 6 bof the grip end 6 of the grip loop is shown again. Corresponding to theposition on the underside of the grip section where code 7 has beenembossed, the upper side is textured by a second impression 10 asindicated by 10′ in FIG. 4. In this case the texturing is a simple(narrow) hatching.

[0034] The subsequent impression is made with a specially structuredstamp 10′ exhibiting a corresponding pattern in a surface of a texturedfield.

[0035] The texture changes the surface without applying a covering layerto the surface. No paint or any other covering is laid over the image 8,but the texture of the surface itself is changed (structure, hatching,or any other penetrating shaping process) and this cannot be removed byscraping or otherwise. This texture change is limited to the area wherethe images 8 of the previous impressions 7 are found, for example theregular delineated area 10′ shown in FIG. 4. This area lies within astrengthened edge section 6 c (for example the reshaped edges of thesheet metal loop) which comprises the grip end (section 6).

[0036] Refer to the cross-section in FIG. 4a of section 3 a′ for anexplanation of the surface change. The right half of the image showswhere the first impression 7 has been applied which formed a definiteimage 8 a′, 8 a″ on the reverse side, i.e. the outer surface 6 b. Theleft half-image of FIG. 4a shows the surface change on the outside (6 b)corresponding to those of FIG. 3. Here we have a texture in the shape ofstructure 11 (which will be explained further in terms of FIG. 5). Thedepth of penetration t₁₁ into this surface is shown.

[0037] The depth of penetration t₇ of the first impression 7 a′, 7 a″ isobvious. The cross-sectional structure of the sheet metal is changed andan image is pressed through on the other side. This image could eitherbe raised or it is visible through an otherwise smooth surface becauseof the altered internal structure. For covering or obscuring the visibleimage 8 a′, 8 a″ corresponding to 8 a in FIG. 3, a second surface changeis required on the side of the image. This is firmly set into thesurface and it alters its integrity, smoothness, or quality in the sensethat disorder is caused in the area concerned by means of directlyinduced surface changes. The depth of penetration t₁₁ which haspreviously been described as obscuring the image is essentially lessthan t₇.

[0038] The ratio between the two depths of penetration is that thestronger first impression 11 on side 6 b is still visible on side 6 a.However, the image 8 a′, 8 a″ formed previously cannot be recognized anymore, having been hidden by texture 11.

[0039] The second surface change is not only less deep, but it is alsomore surface oriented than the first one. The first impression islocalized more strongly to make the mark recognizable. The secondalteration is restricted to the surface to hide the mark. However, bothaffect the surface immediately and directly with the result that theyare fixed and joined permanently with the area involved. Seen from oneside (6 a), the mark is obvious, but from the other side (6 b) it isinvisible. We are mainly concerned with sheet metal parts, but it canalso be applied to other materials like plastic or any other substanceswhose surface structure can be reshaped. The concept of impression canbe adapted to any suitable process by keeping the operation of the firstand second penetrating intrusions in mind.

[0040] The texturing (structuring or hatching) can also be formed inother ways than those shown in FIG. 4. Several examples of impressingsurfaces of a stamp 20 are shown in FIG. 5. At 11 a serrated texture isembossed. In section 12 a texture consisting of fine small tightlypacked cross impressions is shown. At 13 a cross hatching is shown whichappears negative after the second impression. All textures of FIG. 5 aredepicted in combined form, and, when applied, they would cover theentire area 10″ to change the surface structure of side 6 b. The extentof boundary 10″ is calculated to include the three images 8 a, 8 b, and8 c and to form a regular boundary which has already approached aquadrilateral with boundary 10′ as explained in FIG. 4.

[0041] Instead of positioning the code and the second impression on anarea 3 a′ of the grip loop as shown in FIGS. 2 to 4, the lid tab 3 a canalso carry a corresponding embossed code, but in this case also theappropriate area of the outer side of the lid must be textured orhatched by means of a second impression.

[0042] This process is applicable in any area where it is intended toobscure a code or mark impressed on a thin surface section.

[0043]FIG. 6 shows an example of a code 7, 7′, 7″ from the visible side.The image 8, 8′, 8″ appears in FIG. 7. We have an example of a roundtexture impression 11 (similar to the pattern of FIG. 5) in FIG. 8 stillwithout the underlying code numbers 8 being impressed.

[0044] The “enciphered” impression follows from a combination of FIG. 7with FIG. 8. Texture 11 is provided in a limited external area 10′″which extends over the maximum width of codes 7, 7′, 7″. The boundary isregular and differs from the shape of the impressed lines of sevensarranged in a circle.

1. A method for providing a hidden mark (on) a thin metal sheet,preferably a beverage can, wherein a visible and legible mark (7) isprovided on one side (6 a) of the sheet metal by embossing; the reverseside (6 b) of the metal sheet provided with such marking, issubsequently impressed (10,11,12,13) in such a way that the visibilityand legibility of the impressed mark (7) on the one side is retained,but the mirror image (8) appearing on the reverse side by the firstimpressing is covered by the subsequent impression (10), especiallyobscured.
 2. Method according to claim 1, characterized by the image (8)of the first impression (7) being subsequently covered on the reverseside by a textural or hatched second impression on that side after thefirst impression.
 3. Method according to claims 1 or 2, characterized bythe result that the first impression on the inner side of the lid (3)tab (3 a) being covered or made illegible by impressing (10) thecorresponding area of the lid tab from the outside of the lid.
 4. Methodaccording to claims 1 or 2, characterized by the image of the firstimpression (7) on the underside (6 a) of the grip of a grip loop (5)made of thin sheet metal for tearing out the tab (3), being covered by asecond impression on the outside (6 b) of the grip area (6).
 5. Methodaccording to claim 1, in which the sheet metal is so thin that the firstimpression (7 a, 7 b, 7 c; 7) forms an image (8 a, 8 b, 8 c; 8) visiblefrom the reverse side even if this is not embossed or raised.
 6. Methodaccording to claim 1, in which the second impression causes a change inthe surface in a restricted area (10′,10′″,10″) which reduces thesmoothness or quality of the surface compared to sections adjoining theimage (8) which have been untouched by the first impression.
 7. Methodaccording to claim 1, in which the first impression forms a mark (7 a,7′,7 a′) which penetrates the surface by means of carving, tearing orany other method.
 8. Method according to claim 1 or claim 7 in which thefirst impression more clearly penetrates and changes the surface in amore strongly localized area (7 a′,7 a″) than the subsequent impressionon the reverse side (6 b) which extends wider (10′) and alters thesurface to a smaller depth.
 9. Method according to claim 1 in which thesubsequent impression forms a point-pattern (11) in a limited field(10′″) on the reverse side.
 10. A thin sheet metal portion of a can (1),like the body of the can, its bottom, the lid (2) or tear-out tab (5)for attaching to a tear-out lid (2), wherein (i) a visible, legible markor identification (7) is embossed on an interior face (6 a) by a firstimpression, especially by means of scoring; (ii) an image (8,8′,8″) ofthe first impression appearing on the outer face (6 b) is covered by asurface impression (10;11;12;13), for at least substantiallycomplicating legibility.
 11. A part of thin sheet material, especiallylid (2) or grip loop (5) for a sheet metal can according to claim 10,characterized by the outer image (8) of an interior mark (7,7′,7″) beinghidden, is surface covered by a textured, patterned or hatchedsubsequent impression in the sense of an operation changing thestructure of the surface (10;11;12;13).
 12. Metal sheet portionaccording to claim 10 the difficult legibility on a sheet metal partmeans that it is actually illegible.
 13. Metal sheet portion accordingto claim 10 or claim 11, the sheet metal part is a section of a beveragecan.
 14. A proc ss for providing hidden marking of a surface, especiallya sheet metal surface of a can for drinking liquids (beverage), in whichprocess a visible mark (7) is made by means of a strongly penetratingfirst impression on one side (6 a) in at least a limited section of thesurface; a second weakly penetrating impression is made on the reverseside (6 b) which alters the surface (10; 11; 12; 13) to such an extentthat the visibility of the mark (7) made by the first impression on onethe side is retained, an image (8) formed by the first impression on thereverse side, is hidden by the second impression (10; 11, 12) and madeessentially illegible.
 15. Method according to claim 1 or claim 14 inwhich the sheet metal is thinner than 0.3 mm.
 16. A portion of the sheetmetal obtainable by a method according to claim 1 or claim
 14. 17. Metalsheet portion ccording to claims 10, 11, or 16, a thin metal portion (3a′) of the grip (6) is provided with a tear-out loop (5) which is maderigid on at least three sides by means of a shaped edge (6 c). The firstand second impressions lie on opposite sides of the respective surface.18. Method according to claim 1 or claim 14, in which the depth ofpenetration of the first impression is greater than that of the secondone on the reverse side, i.e. the depth of penetration (t₁₁) of thesecond impression is less than that (t₇) of the first impression.